Mizuto and Yume used to date, but now they've suddenly become stepsiblings. Both of them agree that they don't want to do anything that'd disappoint their newly remarried parents. However, neither of them can accept the other as being the eldest sibling in the relationship. So they both agree to settle the matter with a game. Whoever does something in conflict with what people might consider normal sibling behavior, loses. Making them the younger sibling.
Mizuto-kun, are you the kind of bad boy
who'd sneak a peek at his stepsister
أنّك يا ميزوتو-كُن، من الصّبية القذرين الّذين
يختلسون النّظر على أخواتهم غير الشّقيقات
dass du von deiner
Stiefschwester erregt bist,
No serás tan pervertido
como para mirar a tu hermanastra
que tu es un vilain garçon
qui voit sa belle-sœur
sei uno di quei ragazzacci che
danno sbirciatine alle proprie sorellastre
Mizuto-kun, você é do tipo safadinho
que joga olhares indecentes
неужели ты из тех плохих мальчиков
которые смотрят на своих сводных сестёр